[−][src]Struct qt_core::QSemaphore
The QSemaphore class provides a general counting semaphore.
C++ class: QSemaphore
.
The QSemaphore class provides a general counting semaphore.
A semaphore is a generalization of a mutex. While a mutex can only be locked once, it's possible to acquire a semaphore multiple times. Semaphores are typically used to protect a certain number of identical resources.
Semaphores support two fundamental operations, acquire() and release():
- acquire(n) tries to acquire n resources. If there aren't that many resources available, the call will block until this is the case.
- release(n) releases n resources.
There's also a tryAcquire() function that returns immediately if it cannot acquire the resources, and an available() function that returns the number of available resources at any time.
Example:
QSemaphore sem(5); // sem.available() == 5
sem.acquire(3); // sem.available() == 2 sem.acquire(2); // sem.available() == 0 sem.release(5); // sem.available() == 5 sem.release(5); // sem.available() == 10
sem.tryAcquire(1); // sem.available() == 9, returns true sem.tryAcquire(250); // sem.available() == 9, returns false
A typical application of semaphores is for controlling access to a circular buffer shared by a producer thread and a consumer thread. The Semaphores Example shows how to use QSemaphore to solve that problem.
A non-computing example of a semaphore would be dining at a restaurant. A semaphore is initialized with the number of chairs in the restaurant. As people arrive, they want a seat. As seats are filled, available() is decremented. As people leave, the available() is incremented, allowing more people to enter. If a party of 10 people want to be seated, but there are only 9 seats, those 10 people will wait, but a party of 4 people would be seated (taking the available seats to 5, making the party of 10 people wait longer).
Methods
impl QSemaphore
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pub unsafe fn acquire_1a(&mut self, n: c_int)
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Tries to acquire n
resources guarded by the semaphore. If n > available(), this call will block until enough resources are available.
Calls C++ function: void QSemaphore::acquire(int n = …)
.
Tries to acquire n
resources guarded by the semaphore. If n > available(), this call will block until enough resources are available.
See also release(), available(), and tryAcquire().
pub unsafe fn acquire_0a(&mut self)
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Tries to acquire n
resources guarded by the semaphore. If n > available(), this call will block until enough resources are available.
Calls C++ function: void QSemaphore::acquire()
.
Tries to acquire n
resources guarded by the semaphore. If n > available(), this call will block until enough resources are available.
See also release(), available(), and tryAcquire().
pub unsafe fn available(&self) -> c_int
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Returns the number of resources currently available to the semaphore. This number can never be negative.
Calls C++ function: int QSemaphore::available() const
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pub unsafe fn new_1a(n: c_int) -> CppBox<QSemaphore>
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Creates a new semaphore and initializes the number of resources it guards to n (by default, 0).
Calls C++ function: [constructor] void QSemaphore::QSemaphore(int n = …)
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pub unsafe fn new_0a() -> CppBox<QSemaphore>
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The QSemaphore class provides a general counting semaphore.
Calls C++ function: [constructor] void QSemaphore::QSemaphore()
.
The QSemaphore class provides a general counting semaphore.
A semaphore is a generalization of a mutex. While a mutex can only be locked once, it's possible to acquire a semaphore multiple times. Semaphores are typically used to protect a certain number of identical resources.
Semaphores support two fundamental operations, acquire() and release():
- acquire(n) tries to acquire n resources. If there aren't that many resources available, the call will block until this is the case.
- release(n) releases n resources.
There's also a tryAcquire() function that returns immediately if it cannot acquire the resources, and an available() function that returns the number of available resources at any time.
Example:
QSemaphore sem(5); // sem.available() == 5
sem.acquire(3); // sem.available() == 2 sem.acquire(2); // sem.available() == 0 sem.release(5); // sem.available() == 5 sem.release(5); // sem.available() == 10
sem.tryAcquire(1); // sem.available() == 9, returns true sem.tryAcquire(250); // sem.available() == 9, returns false
A typical application of semaphores is for controlling access to a circular buffer shared by a producer thread and a consumer thread. The Semaphores Example shows how to use QSemaphore to solve that problem.
A non-computing example of a semaphore would be dining at a restaurant. A semaphore is initialized with the number of chairs in the restaurant. As people arrive, they want a seat. As seats are filled, available() is decremented. As people leave, the available() is incremented, allowing more people to enter. If a party of 10 people want to be seated, but there are only 9 seats, those 10 people will wait, but a party of 4 people would be seated (taking the available seats to 5, making the party of 10 people wait longer).
pub unsafe fn release_1a(&mut self, n: c_int)
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Releases n resources guarded by the semaphore.
Calls C++ function: void QSemaphore::release(int n = …)
.
Releases n resources guarded by the semaphore.
This function can be used to "create" resources as well. For example:
QSemaphore sem(5); // a semaphore that guards 5 resources sem.acquire(5); // acquire all 5 resources sem.release(5); // release the 5 resources sem.release(10); // "create" 10 new resources
pub unsafe fn release_0a(&mut self)
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Releases n resources guarded by the semaphore.
Calls C++ function: void QSemaphore::release()
.
Releases n resources guarded by the semaphore.
This function can be used to "create" resources as well. For example:
QSemaphore sem(5); // a semaphore that guards 5 resources sem.acquire(5); // acquire all 5 resources sem.release(5); // release the 5 resources sem.release(10); // "create" 10 new resources
pub unsafe fn try_acquire_1a(&mut self, n: c_int) -> bool
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Tries to acquire n
resources guarded by the semaphore and returns true
on success. If available() < n, this call immediately returns false
without acquiring any resources.
Calls C++ function: bool QSemaphore::tryAcquire(int n = …)
.
Tries to acquire n
resources guarded by the semaphore and returns true
on success. If available() < n, this call immediately returns false
without acquiring any resources.
Example:
QSemaphore sem(5); // sem.available() == 5 sem.tryAcquire(250); // sem.available() == 5, returns false sem.tryAcquire(3); // sem.available() == 2, returns true
See also acquire().
pub unsafe fn try_acquire_2a(&mut self, n: c_int, timeout: c_int) -> bool
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Tries to acquire n
resources guarded by the semaphore and returns true
on success. If available() < n, this call will wait for at most timeout milliseconds for resources to become available.
Calls C++ function: bool QSemaphore::tryAcquire(int n, int timeout)
.
Tries to acquire n
resources guarded by the semaphore and returns true
on success. If available() < n, this call will wait for at most timeout milliseconds for resources to become available.
Note: Passing a negative number as the timeout is equivalent to calling acquire(), i.e. this function will wait forever for resources to become available if timeout is negative.
Example:
QSemaphore sem(5); // sem.available() == 5 sem.tryAcquire(250, 1000); // sem.available() == 5, waits 1000 milliseconds and returns false sem.tryAcquire(3, 30000); // sem.available() == 2, returns true without waiting
See also acquire().
pub unsafe fn try_acquire_0a(&mut self) -> bool
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Tries to acquire n
resources guarded by the semaphore and returns true
on success. If available() < n, this call immediately returns false
without acquiring any resources.
Calls C++ function: bool QSemaphore::tryAcquire()
.
Tries to acquire n
resources guarded by the semaphore and returns true
on success. If available() < n, this call immediately returns false
without acquiring any resources.
Example:
QSemaphore sem(5); // sem.available() == 5 sem.tryAcquire(250); // sem.available() == 5, returns false sem.tryAcquire(3); // sem.available() == 2, returns true
See also acquire().
Trait Implementations
impl CppDeletable for QSemaphore
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Auto Trait Implementations
impl RefUnwindSafe for QSemaphore
impl Send for QSemaphore
impl Sync for QSemaphore
impl Unpin for QSemaphore
impl UnwindSafe for QSemaphore
Blanket Implementations
impl<T> Any for T where
T: 'static + ?Sized,
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T: 'static + ?Sized,
impl<T> Borrow<T> for T where
T: ?Sized,
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T: ?Sized,
impl<T> BorrowMut<T> for T where
T: ?Sized,
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T: ?Sized,
fn borrow_mut(&mut self) -> &mut T
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impl<T, U> CastInto<U> for T where
U: CastFrom<T>,
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U: CastFrom<T>,
impl<T> From<T> for T
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impl<T, U> Into<U> for T where
U: From<T>,
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U: From<T>,
impl<T> StaticUpcast<T> for T
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unsafe fn static_upcast(ptr: Ptr<T>) -> Ptr<T>
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unsafe fn static_upcast_mut(ptr: MutPtr<T>) -> MutPtr<T>
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impl<T, U> TryFrom<U> for T where
U: Into<T>,
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U: Into<T>,
type Error = Infallible
The type returned in the event of a conversion error.
fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>
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impl<T, U> TryInto<U> for T where
U: TryFrom<T>,
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U: TryFrom<T>,